HVDC is used for transmissions, and can be used either directly for DC-transmission, or indirectly for regulating AC-transmission systems. Basically, a HVDC system includes a Converter (VSC) for converting AC to DC and vice versa. HVDC converters can be equipped by either thyristors or switchable semiconducting elements, i.e. valves that in contrast to thyristors can be turned off. Examples of turn-off semiconductors are GTO (Gate Turnoff Thyristor), IGBT (Insulated Gate Bipolar Transistor), FET (Field-Effect Transistor), BJT (Bipolar Junction Transistor). HVDC converters with turn-off semiconductors, e.g. HVDC Light converters provided with IGBTs, can also be used for Static VAr Compensation (SVC) and Power Compensation (RPC).
HVDC Light comprises basically two elements: a converter station and a pair of cables.
The converter station includes a control system and Voltage Source Converters. The control system normally controls the VSCs automatically, without communication with other converter stations. A HVDC Light converter station may comprise six switching elements, two switching elements for each phase, comprising high-power transistors in the form of IGBTs (Insulated Gate Bipolar Transistor). The control system, in HVDC Light, is computerised and controls the IGBT-valves by Pulse Width Modulation technique (PWM).
Converter stations having six switching elements in a bridge configuration are called two level converters. An alternative to the two-level converter is the three-level converter having 18 switching elements in a bridge configuration. Also, multi-level converters have been constructed and the present invention primarily concerns the multi-level type.
In HVDC Light, the control system of the Voltage Source CONVERTER station uses Pulse Width Modulation for controlling the IGBT-valves. The modulation is provided by means of generating a tri-signal (a triangular wave), which is compared to a reference signal of the modulation.
In HVDC Light, the control system provides a reference voltage signal (Vref) and a carrier wave, in the form of a triangular carrier signal (tri). The control system compares the two signals and when the reference signal is larger than the carrier signal a switching pulse is generated.
U.S. Pat. No. 7,321,500, B2 (D1) describes a voltage source converter suitable for HVDC Light containing valves, or switching elements, in the form of a plurality of extinguishable (i.e. of turn-off type) semiconducting elements, and a valve control unit including a computer and a pulse width modulator providing an executing control signal for controlling the semiconducting elements (see abstract). The document describes two different modulation methods, the Optimized Pulse Width Modulation (OPWM) and the carrier based Pulse Width Modulation method (carrier based PWM). A drawback with the carrier based PWM is that it requires a higher switching frequency than OPWM. OPWM requires less switching and avoids switching at high current. A drawback with OPWM is that it is vulnerable to transients (column 2, line 9-17 and line 44-65). The voltage source converter of D1 alleviates these problems by using carrier based PWM to handle transients from an AC side and using OPWM otherwise. For this purpose, the VSC includes a “mode detector 14”, and the VSC is adapted to change mode during operation, i.e. between the OPWM and the carrier based PWM (see claim 1, column 9-10).
D1 describes a first and a second VSC station (STN1, STN2) in more detail in FIG. 1, which stations are connected to each other. Each station (STN1, STN2) comprises control equipment (CTRL1, CTRL2) for its respective voltage source converter (CON1, CON2). D1 describes an outer active/reactive power control loop (4) that generates the reference values of converter current, which are inputs for an inner control loop (5). The inner current control loop (5) tracks the reference values of the converter current and generates the voltage reference for the converter. The inner control loop (5) outputs an output signal, which is a voltage reference vector for the bridge voltages of the converter (CON1, CON2). The voltage reference vector is supplied to the pulse width modulation unit (7) that generates a train (FPa, FPb, FPc) of turn on/turn off orders (or pulse train) in accordance with a predetermined PWM pattern supplied to the semiconducting valves. This predetermined PWM is suitably a carrier based PWM, such as a sinusoidal PWM (SPWM) (see column 3-4 of D1). Thus, the control system of the prior art uses a voltage reference signal and a carrier signal to create a switching signal, in the form of a pulse train of switching orders, to the VSC, i.e. to each valve of the VSC.
The present invention can be utilized in such a converter station having outer and inner control loops, but a main concern of the invention is the voltage reference and the generation of switching orders, like a pulse train, and therefore further description of the outer and inner control loops are not described in more detail herein.
The problem concerning the high switching frequency of carrier based pulse width modulation can alternatively be addressed by means of a multi-level-converter.
WO2009/086927 (D2) illustrates a multi-level voltage source converter. A Voltage Source Converter of this type is a development of the type known through for example DE 101 03 031 A1 and WO 2007/023064 A1 and it is referred to as a multi-cell converter or M2LC in D2. D2 argues that multi-level Voltage Source Converter of this type is particularly interesting when the number of the “switching elements” or switching cells is comparatively high, such as at least 8 for each phase, and it may well be in the order of 20. A high number of such switching cells connected in series in said phase leg means that it will be possible to control these switching cells to change between said first and second switching state and already at said phase output obtain an alternating voltage being very close to a sinusoidal voltage. This may be obtained already by means of substantially lower switching frequencies than typically used in known Voltage Source Converters for example of the type shown in FIG. 1 in DE 101 03 031 A1 that includes switching elements with at least one semiconductor device of turn-off type and at least one free wheeling diode connected in antiparallel therewith. This makes it possible to obtain substantially lower switching losses. D2 proposes using a Pulse Width Modulation with a saw toothed, or triangular, wave as a carrier wave for switching when the carrier crosses a reference voltage signal. It has turned out that this method of controlling the switching cells according to a Pulse Width Modulation pattern, with distributed individual saw tooth voltages and individual reference alternating voltages adapted to the actual voltage across said energy storing capacitor of the respective switching element, results in a robust and fast control.
A control arrangement (13) is arranged for controlling the switching cells (7) and by that control the converter to convert direct voltage into alternating voltage and conversely.
The control arrangement (13 of D2) provides a method intended for control of Voltage Source Converters with switching cells (7) of the type having at least two semiconductor devices of turn-off type, at least two free wheeling diodes connected in parallel therewith and at least one energy storing capacitor, and two examples of such switching cells are shown in FIG. 2 and FIG. 3. Note that D2 uses the expression “switching element” for switching cells having two semiconductors (IGBTs), whereas other documents refer to one IGBT as one switching element. Therefore, the expression switching cell is primarily used herein for the combination of two semiconductors together with two diodes. The terminals (14, 15 in D2) of the switching cell are adapted to be connected to adjacent switching cells in the series connection of switching cells forming a phase leg. The semiconductor devices (16, 17) are IGBTs connected in parallel with diodes (18, 19). An energy storing capacitor (20) is connected in parallel with the respective series connection of the diodes and the IGBTs. One terminal (14) is connected to the mid point between the two IGBTs (16, 17) as well as the mid point between the two diodes (18, 19). The other terminal (15) is connected to the energy storing capacitor (20), in one embodiment (the embodiment of FIG. 2 of D2), to one side thereof and in another embodiment (the embodiment according to FIG. 3), to the other side thereof. It is pointed out that each semiconductor device and each diode as shown in FIG. 2 and FIG. 3 of D2 may be more than one connected in series for being able to handle the voltages to be provided, and the semi-conductor devices so connected in series may then be controlled simultaneously so as to act as one single semiconductor device. Also, the switching cells of the present invention comprise serially connected semiconductors that are switched simultaneously, acting as one switch.
D2 describes simulations that were done for a phase leg of a converter (according to FIG. 1 of D2) for a pulse number of 3.37 and a frequency of 50 Hz of the AC voltage. The pulse number is number of switching pulses for a switching cell during one period of the AC voltage. The simulations in D2 propose using a non integer as pulse number, and the non integer pulse number provides a balancing effect of the voltages across the capacitors of the different switching cells. In this way a low pulse number can be selected. The system of D2 can use a pulse number of 3.37, and the present invention is also suitable for low pulse numbers, i.e. non integers less than 10, such as less than 5 or even 3.37 as in D2.
WO2009/087063 (D3) is a development of D2, of the same patent family, and describes a control system of a multilevel converter. D3 discuss problems of methods described in M. Glinka and R. Marquardt, “A new ac/ac multilevel converter family,” IEEE Transactions on Industrial Electronics, vol. 52, no. 3, June 2005, pp. 662-669. Glinka et. al. propose to control the switching of the power electronic switches in a centralized fashion for all submodules in a certain arm. When the control system determines that a switching event should be performed, the submodule (cell) to be switched is selected.
D3 provides a method and system based on the recognition of the fact that the known control system with a central control unit and arm control units has the disadvantage that each arm control unit needs to control the switching of all its corresponding submodules or cells, which has to be done individually, i.e. each cell requires its own input or reference values to be generated by the arm control unit. Please note that D3 refers to the switching cells as “submodules”, whereas D2 uses “switching elements”.
The basic principle of the control method in D3 is to perform switching of the switching cells according to a pulsewidth modulation (PWM) in a distributed manner rather than in a centralized manner, where one of two PWM related signals, a reference AC voltage or a switching carrier signal, is distributed over time.
The reference AC voltage is a reference value for each voltage source and thereby for each arm. The switching carrier signal is identical in shape for all switching cells of each arm.
The necessary delay in time, for each individual switching cell, is applied to the carrier signal by each subunit, or cell control unit, individually, before comparing the carrier and the reference signals with each other.
A saw tooth signal, or triangular signal, is also preferred as carrier signal in D3.
Regarding the physical implementation of the control system of D3, different possible setups are suggested. The cell control units can for example be hardware integrated with the switching cells, but they can as well be placed in a distance to the corresponding switching cell and connected to it by an optical fibre cable or another suitable communication connection. The cell control units may, for performing the data processing, comprise either a digital signal processor (DSP), a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), or a combination thereof. The central control unit may communicate with all the cell control units using electronic circuitry and/or optical fibre cables. In D3, digital signalling should normally be used, with serial and/or parallel communication.
Upon implementing a control system providing a reference voltage signal and a distributed switching control in the cell control units, some design issues arises, especially when controlling switching of a multilevel converter with very low pulse number, like 3.37 pulses described in D2 and D3. The present invention addresses such issues and discloses arrangements and methods to counteract related negative influences.
For these purposes, the present invention provides a voltage source converter station including a voltage source converter and a control system.